Most propeller aircraft have 2, some 3 blades. I guess for the very same reason :=)
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GeorgJun 17 '11 at 11:36

The weight of a blade in a wind generator is probably pretty heavy, this is a part of the equation I guess..
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BjornWJun 17 '11 at 12:31

Yep, but smaller blades would be also much lighter, with a weight factor even bigger than scale factor...
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BarsMonsterJun 17 '11 at 13:05

5

More interesting than number of blades I find the question why that blades cover only a small fraction of the circle they span. (At least for propellers in air). Propellers of ships or Kaplan turbine propellers cover almost the entire circle. (Independent of number of blades)
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GeorgJun 18 '11 at 9:14

3 Answers
3

More blades give you more cost, but very little increase in efficiency.

Three blades turns out to be the optimum.

With four or more blades, costs are higher, with insufficient extra efficiency to compensate. Edit: prompted by a comment, here's some elucidation - this is more expensive per unit electricity generated, if you go for more, but shorter, blades: if you have 4 shorter blades (rather than three longer ones), the blades are sweeping through a smaller volume of air (i.e. an amount of air with a lot less energy), swept area being proportional to the square of the radius. And the efficiency is only a few percent higher.

You get higher mechanical reliability with three blades than with two: with two blades, the shadowing effect of tower & blade puts a lot of strain on the bearings. So although it costs more to make a three-bladed turbine, they tend to have a longer life, lower maintenance needs, and thus on balance reduce the unit cost of electricity generated, as the increased availability and reduced maintenance costs outweigh the extra cost of the third blade.

One thing that helps explain the counterintuitive result: the blade tips are moving at a multiple of the wind speed, so three blades really does sample a decent fraction of the wind flow through the swept area.
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Omega CentauriJun 17 '11 at 16:16

@Omega, I see, this makes sence now. Increasing number of blades would allow to just decrease rotation speed with the same power => lower drag, and that's it. Now it's clear.
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BarsMonsterJun 18 '11 at 22:54

I believe number of blades should also be in this formula. It is hard to expect that theoretically 4 blade turbine would not be able to deliver double what 2 blade one can. So for the same power one may need 30% smaller diameter. Blade weight reduces roughly(i believe) as a cube of it's length, so 4 blades with 30% less diameter should weight 70% of what 2 bigger blades weight.
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BarsMonsterJun 17 '11 at 14:48

I don't think blade weight can reduce as cube of its length. Factoring in issues like mechanical stress and maintaining similar "area per length" for the wind to push on, I would guess the blade weight to scale somewhere between linear and quadratic in length. So I don't really think you can count on there being much gain from weight reduction.
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Willie WongJun 17 '11 at 16:19

I am not an aerodynamical engineer, but off the top of my head there are several additional factors worth considering

Economics The optimisation maybe based on most kWh capacity per dollar invested, which would put additional constraints.

Mechanical stress of the tower More area presented to the wind means more stress on the tower holding the turbine up. Furthermore, presumeably there are issues related to how frequently the blade passes in front of the tower.

Turbulence/wake the closer the various blades are together, the more likely small scale effects like turbulence and wake of the adjacent blades will effect the aerodynamics. Turbulence, for example, can lead to inefficiencies and also differential stress along the body of the blade (compared to laminar flow). Having a few slow-moving, long blades can therefore be a plus to having a lot of fast-moving, short blades. (for the same mass/area/force, many short blades have smaller angular momentum and hence will spin faster in the end, compared to few long blades)

Mechanical stress on the blades to fit more blades at the hub, each blade would have to be thinner in cross section (or you can make the hub really big, but that would just present a huge area for the wind to strike that does no work), so mechanically you probably cannot fit too many blades on the turbine.